The invention relates to an NMR spectrometer with an NMR magnet coil, of which at least one section comprises windings of conductors having a superconducting structure, the structure comprising a plurality of band-segments of band-shaped superconductors, each band-segment comprising a substrate and a superconducting layer deposited thereon, and the band-segments being jointed to each other. The superconducting magnet coil windings are made from a superconducting structure with linked band-segments, which are each overlapped by directly sequential, further band-segments.
Superconductors can carry electrical currents practically without any ohmic losses. They are especially deployed where high electrical currents are required, for example, in magnet coils.
Superconductors can carry loss-free electrical currents up to a critical current density only below a critical temperature (also called transition temperature) and below a temperature-dependent critical magnetic field strength that may also be direction-dependent. Above these critical parameters, i.e. critical current density, critical temperature and critical magnetic field strength which depend on each other, the superconductor enters a normally conductive state.
Metal superconducting materials, such as NbTi, which can be processed as wires, have a relatively low critical temperature (for NbTi, for example, it is about 9K), making their use quite expensive, especially with respect to the necessary cooling. Moreover, metal superconductors have relatively low critical magnetic fields, above which they lose their superconductive properties.
High-temperature superconductors (HTSL), such as yttrium barium copper oxide (YBCO), have significantly higher critical temperatures, YBCO for example, about 90K, but are difficult to process due to their ceramic properties. If they are used at temperatures significantly below their critical temperature, HTSLs can conduct comparatively large currents, i.e. they have a high critical current density. With their comparatively high critical magnetic field strengths, these materials are also suitable for low operating temperatures in strong magnetic fields, as for example desired in NMR spectrometers, in order to achieve high spectral resolution.
For technical applications, HTSLs are usually deposited as a thin layer onto band-shaped, usually metal substrates, wherein, as a rule, one or more buffer layers are interposed between the substrate and the superconducting layer, and one or more final metal layers are deposited on top of the superconducting layer. This type of construction is also termed a band-shaped superconductor and has commonly become known as a “coated conductor”.
However, depositing superconducting layers of good quality is relatively difficult. As a rule, substrate surfaces with a special texture are required, which can only be provided over limited lengths. Currently, good-quality band-segments of band-shaped superconductors are limited to a maximum range of approx. 100 to 500 m. As a consequence, the size of coil sections that can be wound in magnet coils is limited, i.e. in NMR spectrometers. A connection of band-segments by means of joints in a magnet coil is technologically difficult and generally increases the drift of the magnet coil, which is in particular undesirable in the field of high resolution NMR spectroscopy.
EP 0 545 608 A2 proposes joining conductor segments, which can only be manufactured in good quality in limited lengths, in order to enable current to be transported over longer distances, for instance, several kilometers.
US 2005/0173679 A1 discloses the joining of two band-segments of band-shaped superconductors, wherein the superconducting layers on the respective substrates face each other. The superconducting layers are to be in superconducting contact and the proximity of the two polycrystalline superconducting layers is intended to increase the effective grain boundary surface and thus improve the critical current.
U.S. Pat. No. 6,828,507 B1 also discloses the joining together of two band-segments of band-shaped superconductors, wherein the superconducting layers on the respective substrates face each other. The superconducting layers are joined by means of one or more normally conducting intermediate layers (for instance, covering layers of the individual band-segments).
U.S. Pat. No. 6,121,205 A discloses superconducting structures comprising a plurality of overlappingly connected superconductor units, the superconductor units each comprising a substrate consisting of a metal oxide crystal, e.g. of ZrO2 or MgO, and a superconducting layer, e.g. of ReBCO material. In the superconducting structures, the superconducting layers of the superconductor units face each other.
US 2003/0213611 A1 discloses a superconducting structure consisting of a plurality of overlappingly connected bar-shaped superconductor units of solid YBCO. The bar-shaped superconductor units may in particular be arranged in three stacked planes and be connected along a straight direction. In one described embodiment, the superconductor material forms a thin layer on a single-crystalline MgO substrate.
DE 600 31 784 T2 discloses an arrangement of two elements coated by HTS, which elements are connected to each other with their HTS layers facing each other via metallic cover layers. The HTS covered elements may be arranged such that they are slightly offset with respect to each other in a direction transverse to their direction of longitudinal extension.
The object of the invention is to provide an improved NMR spectrometer, in particular with a magnet coil of the spectrometer being able to generate particularly high magnetic field strengths within a sample volume and having a low drift. In particular, an NMR spectrometer shall be provided wherein the magnet coil has a high current carrying capacity in high magnetic field strengths and may, during operation, be short-circuited via a switch and is suitable to generate a homogeneous, temporally extremely stable magnetic field within a sample volume inside the magnet coil, i.e. such that in persistent mode operation a drift of the generated magnetic field caused by residual resistances of the conductors of the magnet coil, of the joints between its sections and/or of the switches stays within tolerable limits, e.g. for high resolution NMR spectroscopy.